This invention relates to a method and an apparatus for forming an image by utilizing electrophotography and, more particularly, to a method and an apparatus for forming an electrophotographic image without using corona discharge means.
A conventional image forming method using corona discharge means includes the steps of uniformly charging a surface of a sensitive medium by a corona discharge means to set a predetermined polarity, exposing the charged surface of the sensitive medium to a light image to selectively disperse the charges on the sensitive medium so that an electrostatic latent image is formed, supplying toner to the surface of the sensitive medium by a development sleeve to which a suitable bias is applied so that the electrostatic latent image formed on the sensitive medium is developed, and transferring the developed toner image on the sensitive medium onto a receptor sheet by using transfer corona discharge means.
This conventional method entails the problem of an offensive smell of ozone generated by corona discharge and the problem of a toxicity of ozone to the human body. This method also entails the problem of the quality of a formed copied image being easily influenced by environmental conditions, i.e., humidity, existence of dust, and other factors.
For the purpose of solving these problems, an image forming method has been proposed in which a charging roller or a transfer roller to which an external voltage is applied is used instead of corona discharge means.
In a known method of this kind, an electroconductive base of a sensitive medium is grounded, and a charging roller to which a DC bias voltage is applied is brought into contact with a surface of the sensitive medium to charge the surface of the sensitive medium. Then the surface of the sensitive medium is exposed to a light image of an original to be copied, and an electrostatic image corresponding to the original image is thereby formed on the sensitive medium. The electrostatic image is developed by toner carried by a development sleeve connected to a suitable bias supply. The developed toner image is transferred onto a receptor sheet such as a paper sheet by a transfer roller to which a transfer bias voltage is applied. The developing powder remaining on the surface of the sensitive medium after transfer is removed from the surface by a cleaning brush to which a suitable cleaning bias is applied.
This method can solve the above-mentioned problems of ozone. In this method, however, it is difficult to charge the sensitive medium surface uniformly and there is a potential problem of image unevenness or background fog of the resulting copied image.
For the purpose of solving this problem, a method of applying a pulsating voltage to the charging roller has been proposed as disclosed in Japanese Laid-Open Patent application No. 63-149668, for example.
This conventional method enables a reduction in charge unevenness on the sensitive medium surface but does not enable a reduction in the overall size of the apparatus. This method requires high-voltage power sources for units of image forming means, i.e., a power source for the charging roller, a power source for the development bias, a power source for the transfer roller and a power source for the cleaning bias. For this reason, if this method is used, it is difficult to provide a low-cost,compact image forming apparatus.